Conventionally, most of the modulators incorporated in an optical communication system generally utilize a direct modulation system for changing an application current to a semiconductor laser or light emitting diode which is a light source and directly modulating an output light therefrom.
However, in the case of this system, there is a problem that the light property is made unstable according to the modulation of the intensity in an LD (laser diode) generally used as a light source and the transmission distance of an optical signal is considerably limited.
Further, in this system, one signal source (signal transmission source) is used as to correspond to one transmission line, it becomes necessary to insert an optical coupler between the individual signal source and the transmission line in order to couple optical signals composed of a plurality of signal sources into one transmission line.
However, in the above direct modulation system, an insertion loss caused by the insertion of the optical coupler is large. For example, even in a case of a waveguide type optical coupler whose insertion loss is said to be relatively small, it is approx. 0.5 dB. Therefore, the number of signals which can be coupled to one transmission line is limited.
In order to overcome the above limitation in the direct modulation system, recently, a modulator utilizing an external modulation system is developed.
The external modulation system is a system having a D.C. light source whose intensity is constant with time, and modulating a propagation light by use of a modulator arranged on a half-way of the optical transmission line and can exclude obstacles based on instability of the light source. Further, it has an advantage that the insertion loss caused when the modulator is inserted to the optical transmission line is small, and therefore, a large number of modulators can be incorporated into the optical transmission line.
As a modulator incorporated into an optical communication system of the above external modulation system, the following examples are known as representative examples.
One of them is obtained by forming a waveguide pattern having a large refractive index in a substrate made of LiNbO.sub.3 by ion exchanging operation, for example, and disposing an electrode for voltage application near the waveguide.
The external modulator is operated on the basis of the electrooptic effect, and the modulation characteristic in the high frequency region (several GHz) is excellent, but the dependency thereof on variations in the temperature and moisture is large, the dependency of the light output intensity on the bias voltage fluctuates according to deterioration with time and expansion/contraction of the waveguide base plate caused by the above variations, thus providing a problem that it is difficult to attain the stable operation. Further, since the single crystal of LiNbO.sub.3 is extremely expensive and has a difficulty in the practical application in the industry. In the case of this modulator, the insertion loss is as large as 2 to 4 dB, and therefore, it is difficult to dispose a large number of modulators in one optical transmission line.
Further, there is provided such a type as shown in FIG. 1 and disclosed by D. S. Czaplak and F. S. Hickernell in Ultrasonics Symposium, 1987, pp 491 to 493. The external modulator has such a structure that the outer periphery of a clad layer la of a single mode optical fiber 1 is covered with a lower electrode which is formed of a Cr--Au thin film, a thin film 3 formed of ZnO which is a piezoelectric material and an upper electrode 4 which is formed of a Cr--Au thin film.
Further, Godil et al. proposed a structure constructed by laminating a lower electrode 2 formed of Cu/Au and a piezoelectric thin film 3 formed of ZnO, sequentially and partially on a half-portion of the periphery of the optical fiber as shown in FIG. 2 in J. Lightwave Technol vol. 6, 1586, 1988. In the case of the above external modulator, an unnecessary resonance mode of elastic waves can be suppressed not by orientating the stress of the elastic waves concentrated on a core 1b of the optical fiber symmetrically with respect to an axis of the optical fiber, but by orientating the same perpendicular to the axis.
In the above external modulators, since the optical fiber and the external modulating means are formed in an integral structure, there is provided an advantage that the insertion loss in the transmission line is extremely small and they can be manufactured at a low cost.
However, in the case of the above external modulators, the lower electrode, piezoelectric thin film, and upper electrode are formed by applying the vapor deposition method or sputtering method, but since the surface of the optical fiber 1 is a curved surface, it becomes necessary to rotate the optical fiber in the film forming device or use other high-degree operation technology in order to form a piezoelectric film having a uniform piezoelectric characteristic over the entire range of the circumferential direction, thus providing a problem in the manufacturing process.
Further, there is provided a problem of the characteristic that the elastic wave is considerably reflected on the periphery of the optical fiber so as to cause the resonance of the elastic wave inside the optical fiber, and as a result, the frequency characteristic of the modulator cannot be made flat.
Further, D. B. Patterson et al. made public a type as-shown in FIG. 3 in Optics Letters, vol. 14, No. 4, 1989, pp. 248 to 250.
The external modulator has a structure obtained by sequentially laminating a lower electrode 2, a thin film 3 of ZnO and an upper electrode 4 in this order on one-side surface of a substrate 5 made of quartz glass, forming a groove 5a of a semi-circular cross section on another surface, and closely burying a single mode optical fiber 1 in the groove 5a.
Like the modulator proposed by Hickernell et al., the above external modulator has an advantage that since the insertion loss thereof in the transmission line becomes extremely small and the surface of the substrate 5 is flat, the film thickness control for the lower electrode. 2, piezoelectric thin film 3 and upper electrode 4 can be made easy respectively, therefor a uniform characteristic can be obtained over the entire surface.
However, in the case of the above external modulator, it is extremely difficult to work the groove 5a formed in one surface of the substrate 5 of quartz glass so as to exactly coincide with the curvature of the cross section of the single mode optical fiber 1 which is closely buried in the groove. Further, for the same reason as in the case of Hickernell and Godil, the resonance phenomenon occurs in the optical fiber and the frequency characteristic of the modulator is not made flat.
Further, since the velocities of elastic waves (sound waves) propagating in the substrate 5 and the optical fiber 1 are substantially the same, the elastic wave propagates straightforwardly at the portion of the interface between the substrate 5 and the optical fiber 1, thus making it difficult to effectively converge the elastic wave into the core of the optical fiber 1. Particularly, when the driving frequency is high, the directivity of the elastic wave generated in the piezoelectric thin film 3 is extremely sharp and the elastic wave propagates straightforwardly in the substrate 5 so that the amount of the elastic wave which converges into the core of the optical fiber 1 will be extremely limited. That is, the rate at which the elastic wave generated in the piezoelectric thin film 3 contributes to modulation of the light propagating in the optical fiber 1 is extremely small and the efficiency is lowered.
In order to improve above problems, it is effective to increase the length/width ratio of the portion functioning as the piezoelectric element so as to increase the effective length of the piezoelectric thin film for the optical fiber, but when such a process is effected, the width of the piezoelectric thin film 3 or upper electrode 4 becomes narrow so that setting of the positional relation between the piezoelectric thin film and the optical fiber will require high precision, thereby making the setting difficult.
Further, the external modulators proposed by Hickernell, Godil, Patterson have commonly the following problems described hereinafter.
First, the amplitude of the elastic wave generated in the piezoelectric thin film largely depends on the film thickness. The amplitude can be measured based on the frequency applied to the piezoelectric thin film and the S/N ratio, and the value thereof is set to the maximum value at the resonance frequency fr and considerably attenuates as the frequency is deviated from the resonance frequency fr as shown in FIG. 4 so that the bandwidth of the modulation frequency is narrowed accordingly.
Further, in the case of the above external modulators, since the piezoelectric thin film is used as a modulation medium, the piezoelectric element section constructed by the piezoelectric thin film, lower electrode and upper electrode electrically functions as a capacitor. For this reason, the impedance thereof varies with the frequency of a modulation signal from a modulation signal output section which is an information source. In order to effectively transmit the modulation signal to the piezoelectric element section, it is necessary to attain electrical matching between the modulation signal output section which is an information source and the piezoelectric element section.
The impedance variation of the piezoelectric element section based on the frequency depends on the electrical capacitance of the piezoelectric element section and the electrical capacitance largely depends on the area of the upper electrode. Therefore, the frequency band used to the above external modulators depends on the area of the upper electrode. For this reason, in the case of the external modulator having only one upper electrode, there occurs a problem that only one frequency band which is applicable can be set and the amount of information to be transmitted is reduced.
In order to solve this problem, it is considered to serially connect a plurality of external modulators to one station. In this case, when a plurality of external modulators are connected to one another by use of optical fibers, they can be easily connected by use of adapters for connectors if optical connectors are connected to both ends of the optical fiber, but in this connection system, there occurs a new problem that the insertion loss is not negligible when the number of connecting portions becomes large and a light is reflected between the connectors.
Further, if the ends of the optical fibers are connected by fusion method, the above-described problem concerning the insertion loss and reflection of light can be solved, but a problem that the handling thereof is difficult occurs.
Further, as a problem common to the above two connection systems, a problem that the occupied space of the modulation system is large occurs.
Further, there is provided a type made public by Fujisaki et al. in Institute of Electronics and Communication Engineers at National Convention, Spring, B-891, 1990. The external modulator has a structure constructed by mechanically pushing PZT piezoelectric ceramics on a coating of single mode optical fiber.
However, in this type of external modulator, since the piezoelectric element and the clad of the single mode optical fiber are not closely contacted together, the transmission efficiency of the elastic wave from the piezoelectric element to the-optical fiber is degraded, and as a result, it is difficult to say that the modulation for high frequencies higher than 1 MHz can be adequately attained.
An object of this invention is to provide an external modulator for optical communication in which the insertion loss is small and which indicates a flat frequency characteristic having no special resonance peak and can attain the modulation for the high frequency such as approx. 500 MHz.
Another object of this invention is to provide an external modulator for optical communication which can be easily manufactured since the film thicknesses of the lower electrode, piezoelectric thin film and upper electrode can be easily controlled, respectively, and in which the control of orientation of the piezoelectric thin film can be easily attained and a preferable piezoelectric effect can be realized.
Still another object of this invention is to provide an external modulator for optical communication in which elastic waves generated in the piezoelectric thin film can be efficiently converged into the core of an optical fiber so as to attain an excellent modulation efficiency.
Another object of this invention is to provide an external modulator for optical communication in which the frequency characteristic of the output efficiency of elastic waves generated in the piezoelectric thin film is broad and the modulation frequency bandwidth is widened.
Still another object of this invention is to provide an external modulator for optical communication which can transmit a large amount of information with a single external modulator and the occupied space of a modulation system can be made small.